1. Field of the Invention
The invention relates to optical disc drives, and in particular, to an average servo control method and system, and a target level determination method for an optical disc drive.
2. Description of the Related Art
FIG. 1 shows a conventional average servo loop of an optical disc drive. The optical disc drive usually comprises a mechanical module 110, a gain down stage 120, and a post processing stage 130. The mechanical module 110 conclusively comprises a pickup head, a plurality of photo detectors and other mechanical parts such as an actuator (not shown). A laser beam is radiated from the mechanical module 110 to an optical disc (not shown), and reflected light is detected by the photo detectors. The actuator in the mechanical module 110 adjusts focus and track conditions of the laser beam based on the driving signal #drive feedback from the post processing stage 130. The photo detectors in the mechanical module 110 send their photo detection results #light to the gain down stage 120, and the gain down stage 120 converts them into servo signals #servo such as playback signals (RF signals), focus error (FE) signals and push-pull (PP) signals based on a certain gain control mechanism. The RF signals represent information recorded on the optical disc, and the PP signals comprise tracking error (TE) signals and wobble signals. After the gain down stage 120 justifies the gains of the photo detection results #light, the post processing stage 130 receives the servo signals #servo to determine the servo condition and then sends the driving signal #drive to the mechanical module 110.
Conventionally, one of the servo signals #servo such as playback signals (RF signals), focus error (FE) signals and push-pull (PP) signals is selected as an index signal #index. A comparator 122 is coupled to the output of gain down stage 120, providing gain adjustment to the photo detection results #light based on the index signal #index. For example, the total power of the RF signals (also referred to as RF sum), is sent to the comparator 122 as the index signal #index. The comparator 122 compares the index signal #index and a target level #target, whereby their difference is used to adjust the gains of the servo signals. Specifically, a control signal #ctrl is generated from the comparator 122 and fed back to the gain down stage 120 to adjust the gains used in the gain down stage 120. In this way, the RF, FE and PP signals generated from the gain down stage 120 can have constant gain levels approximately identical to the target level #target.
Since servo signals #servo are utilized for servo loop compensation that keeps the focusing and the tracking stable, gain control thereof is critical. The comparator 122 conventionally utilizes a constant target level #target to achieve consistent gains of the servo signals #servo, however, a constant target level #target is considered inadequate for some conditions, particularly when the photo detection results #light varies with track position due to disc dye material dis-uniformity, reflection variation, and spinning speed variation. Thus, failure may occur when performing a write procedure to a disc when using a constant predetermined target level #target. An improved gain adjustment mechanism is thus desirable.